We have moved to PHP 7 and noticed the differences immediatly. We tested HHVM before, but it wasn't stable enough at the time. We have had some small issues with PHP7, but nothing that couldn't be fixed in a few minutes. The issues that existed were basically from either bad coding, or deprecated code. For the codes that were fixed, the information was forwarded to the developer to include in the next updates.
I personally find PHP7 to be a form of comedy for the fact it's calling out bad and lazy coding. Themes and plugins coded properly work on any version of PHP. Other themes and plugins...look at the code the wrong way and things start breaking. A real "let's just throw codes together" and hope it works. Too many stories.